See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/230743857 A Multitracer Study of Radionuclides in Lake Zurich, Switzerland 1. Comparison of Atmospheric and Sedimentary Fluxes of 7Be, 10Be, 210Pb, 210Po, and 137Cs ARTICLE in JOURNAL OF GEOPHYSICAL RESEARCH ATMOSPHERES · SEPTEMBER 1991 Impact Factor: 3.43 · DOI: 10.1029/91JC01765 CITATIONS 50 READS 48 12 AUTHORS, INCLUDING: Erich Wieland Paul Scherrer Institut 101 PUBLICATIONS 2,093 CITATIONS SEE PROFILE Peter H. Santschi Texas A&M University - Galveston 337 PUBLICATIONS 12,657 CITATIONS SEE PROFILE Michael Sturm Eawag: Das Wasserforschungs-Institut des E… 164 PUBLICATIONS 3,774 CITATIONS SEE PROFILE Juerg Beer Eawag: Das Wasserforschungs-Institut des E… 310 PUBLICATIONS 14,179 CITATIONS SEE PROFILE Available from: Erich Wieland Retrieved on: 03 February 2016
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 96, NO. C9, PAGES 17,051-17,065, SEPTEMBER 15, 1991
A Multitracer Study of Radionuclides in Lake Zurich, Switzerland 1. Comparison of Atmospheric and Sedimentary Fluxes
of 7Be, øBe, 2øPb, 2øPo, and 137Cs
c. SCHULER,' E.WIELAND, z3 P.H. SANTSCHI, 4 M. STURM, 2 A. LUECK, 2 S. BOLLHALDER, 2 J. BEER, 2 G.BONANI,SH.J. HOFMANN, s M. SurER,' AND W. WOLFLI s
Atmospheric and sedimentary fluxes of natural (i.e., 2'øpb, 2'øpo, 7Be, and 'øBe) and artificial (i.e., Chernobyl mCs) radionuclides and particles through Lake Zurich (at 50 m and 130 m depth) between 1983 and 1987 were compared in order to establish trr. ce metal pathways and their kinetics. Atmospheric fluxes averaged 0.83 dpm cm -2 yr" for 2'øpb and 16.0 dpm cm '2 yr" for 7Be during 1984 to 1987. Vertical fluxes of 2'øpb and 7Be, associated with settling particles, were measured in sediment traps deployed at 50 m and 130 m depth at a station near the deepest part of Lake Zurich. Average fluxes of 2'øFo and 7Be at 50 m depth were 0.94 and 4.90 dpm cm '2 yr 4, respectively, and 16.5 x 107 atoms m '2 d" for 'øBe. Sediment traps at 50 m quantitatively collected atmospherically deposited 2'øpb falli.ng through the water column. At 130 m depth, immediately above seasonally anoxic bottom waters, 2'øpb, 'øl•e, and'37Cs fluxes were higher than at 50 m by up to 60% at times during the summer stagnation period. Sediment inventories of :mPb and 'øBe are in agreement with atmospheric nuclide fluxes, suggesting only moderate recycling of these nuclides in the bottom waters, whereas mCs inventory in the sediments is lower than expected from sediment trap fluxes, indicating remobilization of this nuclide.
INTRODUCTION
Because of their defined input function, natural and man-made radionuclides make excellent tracers to study the geochemical cycling of elements in surface and groundwaters (for example, see the reviews in the wozk of Santschi and Honeyman [1989] and Murray [1987]).
The radiotracer methodology has firmly established itseft in the marine sciences for the study of mechanisms and rates of processes which control the movement in the water of soluble and particulate trace elements over varying time scales [Broecker and Peng, 1982]. In the limnological sciences, it has become a promising but not yet established tool. However, even though lakes and oceans are similar in some respects, there are also important differences. These arise from the fact that lakes are often more dynamic and more variable than the oceans, thus giving rise to more rapidly changing radionuclide and trace element concentrations in the water. Furthermore, generalizations are often more difficult as different factors such as boundary effects, geographic location, hydrography, or trophic status play an important role in the removal process as well.
A small number of investigators, however, have studied the seasonal cycling of radionuclides in lakes in order to use information on residence times and transport pathways in the investigations of in-lake processes as well as boundary effects. For example, Barnes et al. [1979], Appel [1982], Barth [1984], Talbot and Andren [1984], Stiller and Imboden [1986],
'Swiss Federal Institute of Water Resources and Water Pollution Control (EAWAG), Dubendorf, Switzerland.
:Institut fur Mittel-energiephysik, ETH Honggerberg, Zurich, Switzerland.
•Now at Department of Civil Engineering, Stanford University, Stanford, CA.
4Department of Marine Sciences, Texas A & M University, Galveston.
SPaul Scherrer Institut, Villigen, Switzerland.
Copyright 1991 by the American Geophysical Union.
Paper number 915C01765. 0148-0227/91/91 $C-01765505.00
Benoit and Hemond [1986], and Mangini et al. [1990] used 210pb and 210po measurements to study trace element residence times in lakes as they are affected by boundary processes. Robbins and Eadie [1982], Schiff et al. [1983], and Hawley et al. [1986] used the short-lived 7Be while the group of Edgington, Robbins, and Eadie (for example, see the review of Eadie and Robbins [1987]) used also some fallout radionuclides in the Great Lakes for the same purpose. The purposeful tracer studies of the group of Broecker, Hesslein, Schindler, Emerson, and Santschi allowed similar studies in the Canadian Shield lakes [e.g., Hesslein et al., 1980; Hesslein, 1987; Santschi et al., 1986, and references therein]. Whereas
10Be has recently been analysed on marine particles [Sharrna et al., 1987], we present here for the first time measurements of 10Be concentrations on lacustrine particles. We present this five-year radiotracer study in two parts. In the
first part, vertical fluxes from the atmosphere into the lake and from the lake water to their final repositories, the sediments, are discussed. The aim of this first paper is to establish vertical scavenging efficiencies of transfer from the atmosphere through the lake water into the sediments and to elucidate seasonal controls of radionuclide scavenging. For this purpose, efficient and reliable collectors of vertical fluxes were needed as well as independent means for assessing their accuracy. Another purpose was to investigate the relationships between radioactive 210pb and stable lab in Lake Zurich, as their input functions into the environment as well as their chemical form are different [Salomons and F6rstner, 1984].
In the second part, we will discuss the importance of boundary effects on radionuclide behavior in the interior of a lake and present a conceptual and mathematical model for assessing particle dynamics. Ratios of radionuclide fluxes are used to test the model and to assess the magnitude of boundary effects [Wieland et al., this issue].
The nuclides investigated are 10Be and 7Be (cosmogenic origin, haft-life of 1.5 x 106 years and 53 days, respectively), 210pb (produced by radioactive decay of the soft-derived noble gas 222Rn in the atmosphere, half-life of 22 years), Chernobyl 137Cs (half-life of 30 years), and 210P o (from
17,051
17,052 SCHULER ET AL.: RADIONUCLIDES IN LAKE ZURICH, 1, FLUXES
210pb decay, half-life of 138 days) as near-ideal tracers with quantfriable input functions. We will show that atmospheric precipitation is the most prominent input term for many particle-reactive trace elements and radionuclides (e.g., cosmogenic and radon-produced nuclides). River inputs of these nuclides are negligible for Lake Zurich, because of the presence of two upper lying lakes which remove particles and particle-reactive trace metals and nuclides from the water before they can enter the lower part of Lake Zurich [Santschi ½'t al., 1990].
In situ production of 210pb in lakes with similar mean depth and climate as those of Lake Zurich is generally insignificant [Stiller and lmboden, 1986]. This can be shown as follows. According to D.M. Imboden (personal communication, 1990), 222Rn concentrations in near-bottom waters of Lake Zurich are very low, despite a relatively high measured potential production rate in the sediments. Expected fluxes of 222Rn are about 300 dpm m '2 d' 1 which correspond to a calculated surface water concentration of about 6 x 10 -5 dpm L' 1. The calculation assumes a ratio of water volume in the top 0-10 m of the water column to surface area of boundary sediments of about 50 m. Since the water residence time in the mixed layer is certainly less than the 1-year average water residence time of the lake, the 210pb production from 222Rn decay can be calculated by [210pb] = [222Rn] x (1-exp(-3,pbAt))<10 '3 dpm L '1 which is significantly smaller than the measured activity of 10 '2 dpm L '1 of 210pb in the water.
STUDY Srm
Lake Zurich is U-shaped in cross section and has steep lateral slopes on two sides (east, west) but gentle slopes on the south and north side. The lake is divided by a dam into two basins, the Upper Lake and the Lower Lake. The major inlet into Lake Zurich is the Linth River connecting Lake Walen and the Upper Lake, and the majority of the allochthonous material of the fiver input is deposited in these two preceding lakes. Therefore, lower Lake Zurich which, as the major basin of this lake, is relatively free of riverine sediment inputs during most of the year except in the winter. This feature makes this lake ideal for observing in-lake processes. The lower basin of Lake Zurich has a surface area of 6?.3 km 2, a volume of 3.4 km 3, and a drainage basin of 1829 km 2. Its maximum depth and average depth are 137 m and 49 m, respectively. Lake Zurich is stratified between April and October, with the thermocline varying between a depth of 10 m and a depth of 15 m. The deepest water layer below 130 m becomes anoxic during summertime. Settling material between 50 m and 130 m is composed of organic C (14-60%), CaCO 3 (20-60%), MgCO3 (1-3%), Fe203 (0.2-2%), MnO2 (0.1-16%) and silicates (0.5- 40%) [Sigg et al., 1987]. Sediment traps were deployed at a station near the deepest part of the lake at depths of 50 m and 130 m to collect the particle flux through the water column.
METHOI•
Samples of rainwater obtained from collectors of atmospheric precipitation on the roof of the Swiss Federal Institute of Water Resources and Water Pollution Control (EAWAG) (13 km away from the lake) [$antschi et al., 1988], sediments from collectors of the particle rain in the lake (sediment traps) [Bloesch and Sturm, 1986; Sigg et al., 1987] and from in situ
filtration and extraction (~700 L in 2-6 hours) [Bishop et al., 1985], or water (100 L) pumped from different depths in the lake, were taken to the laboratory.
The collection methods of settling particles in aquatic systems have recently been reviewed [e.g., Butman, 1986; Knauer et al., 1984]. Artifacts can be caused by hydrodynamic effects, which can bias collection efficiencies, and by zooplankton grazing and capture. In lakes below the thermocline, current velocities are usually below a few centimeters per second [Letorain and lmboden, 1987], and therefore one would expect the hydrodynamic artifacts to be minimal. Zooplankton grazing can cause trace element loss to solution as well as extra gain of particulate matter in the collection cups. The reliability of the vertical flux measurements therefore needs to be assessed independently. Atmospherically delivered radionuc!ides are ideal for this purpose.
The problems of atmospheric collectors are discussed by Dasch [1985]. Since we were mostly interested in measuring deposition rates onto a water surface, a cylindrical collector which had some acidified water on the bottom and which was
set up on the rooftop of the laboratory building (EAWAG) was used. The collection efficiency of our atmospheric collectors is probably within 100 + 10% [Santschi et al., 1988], as judged from comparison of the fluxes of various Chernobyl radionuclides in collectors of different positions and geometry, as well as from inventories in grass 3-4 days after fallout [Santschi et al., 1987, 1988]. Rain or water samples were acidified with HCI to pH=l. FeC13 and appropriate yield tracers and carriers (e.g., 208po, stable Pb and Be) were added in order to determine procedural yields. After a suitable time of the order of 1 day, Fe(OH)3 was precipitated by adding NH4OH and was separated. Merck analytical reagent grade chemicals were used to minimize blank effects.
Sediment traps with diameters of 20 cm and lengths of 100 cm, deployed in duplicates, collected between 0.1 and several grams of settling particles within the 14 to 21 days of exposition. No poison was added to these traps in order to avoid contamination. Zooplankton feeding on material collected in these traps was judged not to be a serious problem due to the low temperature (i.e., <6øC), light and productivity levels at depths of 50 m and below [Sigg et al., 1987]. Sediment trap samples were directly gamma-counted in the same vial type, after they were concentrated by freeze-drying. The dry weight was corrected for contribution of dissolved salts resulting from freeze-drying of supernatent water. All manipulations of the particles were carried out in a laminar flow hood. A gravity corer with polyvinylchloride (PVC) tubes was used to collect sediment samples.
The sediment core was taken in 1987 at 130 m water depth, dissected in 0.6- to 1.2-cm intervals within a few hours after
collection, dried at 60øC, weighted, and homogenized before non destructive gamma ray spectroscopy and further chemical analysis. Dry weights were determined from aliquots of fresh sediment samples.
Fe(OH)3 precipitates, freeze-dried particles and dried sediment samples were gamma-counted on a high purity Ge well detector coupled to a Canberra S-90 multichannel analyzer and PDP H- 23+ computer in order to measure 7Be (478 keY), 2!0pb (46 keY) and 137Cs (661 keY). Results were corrected for radioactive decay, ingrowth and yield of extraction using the recoveries of Pb and Be in the final Fe(OH) 3 precipitate, determined by atomic absorption spectromel•. High purity
SCHULER ET AL.' RADIONUCLIDES IN LAKE ZURICH, 1, FLUXES 17,053
well detector geometries were calibrated with standards of the appropriate radionuclide (obtained from New England Nuclear Corporation and Amersham Corporation) in orde, r to circumvent summation corrections. Polonium 210 (and also
210pb via 210po ingrowth) was measured by alpha counting after proper chemical separation [Santschi et al., 1979, 1980] by using isotope dilution methodology. Measured nuclide activities determined by gamma counting were calculated by comparing them to standard solutions of the same nuclide in the same geometry. Standards of 208po (for example, from Amersham Corporation; Health and Safety Laboratory, New York, or courtesy of J. Dominik, University of Geneva, "Goodbye" standard) were calibrated against 210pb solutions supplied from Amersham Corporation and Environmental Protection Agency Laboratories; standard solutions of 7Be were supplied from Laboratoire de Meteorologie des Rayonements Ionisants, Gif-sur-lvette, France and 137Cs was supplied by New England Nuclear Corporation, Dreieich, Germany. Activity measurements are given in units of decays per minute (60 dpm -- 1 Bq).
The 10Be measurements were carried out at the Swiss Federal Institute of Technology/Paul Scherrer Institute (ETH/PSI) accelerator mass spectometry (AMS) facility in Zurich [Suter et al., 1984]. Samples of trapped material (approximately 0.3- 0.6 g) were spiked with 0.5 mg 9Be carrier, subsequently dissolved in 3 mL concentrated HCI, and mixed with 2 mL of
25% H20 2. Then, suspensions were carefully heated up by ]R lamps in order to enhance decomposition of organic matter. Within half an hour, CO 2 release was completed, and the suspensions were agitated ultrasonically for 1 hour. After centrifuging (10 min at 4000 rpm), solutions were transferred into $0-mL beakers. The residual matter was again resuspended in 6 M HCI, agitated ultrasonically for 10 min and centrifuged. A pH of ~2.5 was set by adding 10 M NaOH to combined solutions. After adding 0.5 mL 10% Ethylene Diamine Tetra Acetate (EDTA) solution, the pH was raised to 9.5 using concentrated NH4OH. Solutions were equilibrated overnight and then centrifuged. The white-reddish precipitate was washed several times by resuspending it in 14 M NaOH and by subsequenfiy centrifuging suspensions (10 min at 4000 rpm) in order to reduce the amount of slightly soluble hydroxides (eg., Fe(OH)3). The residual, mainly Be hydroxide, was washed with distilled water, dried at 60øC for 24 hours, and converted to BeO by ignition at 950øC in a quartz crucible [Beer et al., 1983].
RP.s•s Al• DmCUSS•ON
Atmospheric Input Functions Atmospheric fluxes of 7Be and 210pb are shown in Figures
la and lb. The most striking features are the pronounced flux maxima in July/August for both 7Be and 210pb nuc!ides, indicating a similar seasonality of the input functions of both nuclides. This similarity is further documented in Figure lc by the close correlation of the two fluxes extending over 4 years. The correlation coefficient, R, for the correlation of 210pb versus 7Be fluxes is 0.90 (with N--22, this is significant at the 99.9% confidence level). Both nuclides also correlate closely with air temperature (r--0.69 and 0.78 for 210pb and 7Be, respectively) but not with rainfall (Table 5). The close correlation of atmospheric 210pb fluxes and air temperature indicates the seasonality of the two parameters. It might
indicate the temperature control of the flux of 222Rn out of soils, which is also controlled by humidity. If we assume that mixing of stratospheric 7Be into the troposphere is occurring in late spring, the summer maxima of atmospheric 7Be flux most likely reflect seasonal control of deposition by atmospheric precipitation (e.g., summer thunderstorms) and scavenging processes. Concurrent maxima in atmospheric deposition rates of 210pb and 7Be have also been reported by Dominik et al. [1987], Olsen et al. [1985, 1986], and Matsunami et al. [ 1979]. However, their flux maxima occurred, as expected, in springtime. Atmospheric fluxes of 134Cs and 137Cs were taken from
Santschi et al. [1988, 1990] as 4.5 kBq m '2, deposited between April 30 and May 8, 1986. The initial 134Cs/137Cs ratio was 0.50 ñ 0.03.
The average 7Be flux during the years 1984-1987 (16.0 dpm cm '2 yr- 1 at ~ 110 cm yr- 1 rainfall; see Table 1) agrees well with that measured in atmospheric collectors on the shores of Lake Geneva in the western part of Switzerland, i.e., 16.5 dpm cm '2 yr '1 (at 120 cm yr '1 rainfall [Dominik et al., 1987]) and 15.9 dpm cm '2 yr '1 calculated for this latitude band from average atmospheric concentrations of 7Be of 0.18 dpm m '3 [Kommission zur Ueberwachung der Radioaktivitdt (KUER), 1982] and transfer velocities of 2.8 cm s' 1 [Turekian et al., 1983]. Our average 210pb flux for the years !984-1987 is 0.83 dpm cm '2 yr '1 (Table 1), again close to the value determined by Dominik et al. [1987] for the western part of Switzerland (0.91 dpm cm '2 yr' 1) and the value of 0.83 dpm cm '2 yr' 1 expected from the average atmospheric concentration of 0.026 dpm m '3 [KUER, 1982] and the transfer velocity of 0.95 cm s '1 [Turekian et al., 1983]. During 1987, total atmospheric deposition of 10Be and 7Be
averaged to 5.0 + 2.2 x 107 and 2.9 + 1.4 x !07 atoms m '2 d' 1, respectively (total rainfall: 110 cm yr' 1). Deposition fluxes are higher in summer (7Be: 3.7 d- 1.1 x 107 atoms m '2 d '1, 10Be: 6.1 d- 2.5 x 107 atoms m '2 d '1) than in winter (7Be: 1.4 x 107 atoms m '2 d '1 10Be' 2.9 x 107 atoms m '2 d '1) • ß ,
Atmospheric 7Be and 10Be fluxes measured in the western part of Switzerland (Berne) during 1980-1985 by Luder [ 1986] were 3.0 d- 0.3 x 107 atoms m '2 d '1 for 7Be and 8.0 d- 1.2 x 107 atoms m '2 d '! for 10Be (total rainfall: 103 cm yr'l). Since 10Be and 7Be are produced by cosmic rays with a
constant ratio of about 0.5 in the atmosphere, the ratio 10Be/7Be represents an ideal chronometer for transport processes [Raisbeck et al., 1981]. The annual 10Be/7Be ratio measured in atmospheric deposition was 1.86 d- 0.62 in Diibendorf and 2.48 d- 0.32 in the western part of Switzerland [Luder, 1986] . This ratio is higher than the production ratios due to radioactive decay of 7Be in the atmosphere.
Vertical Fluxes of Particles and Radionuclides in Sediment Traps
Sediment trap samples were collected between 1983 and 1987 in Lake Zurich in order to determine particle as well as isotope fluxes (7Be, 10Be, !37Cs, and 210pb). Average sediment trap fluxes of particles, 7Be, and 210pb in
both sediment traps were calculated for summer and winter periods between 1983 and 1987 and are listed in Table 2. Errors
17,054 SCHULER ET AL.' RADIONUCLIDES IN LAKE ZURICH, 1, FLUX,ES
•lOO0
X .9
.8
I
CI3 .7
CD .6
0 .5
.3
.2
a
1983 1964 1965 1966 t967
80-
,--, 70-
x 50-
i •o-
•- 30-
E 20-
l0
i983 1984 1965 1966 1987
Fig. 1. Atmospheric fluxes of (a) 7Be and (b) 210pb as a function of time measured at Diibendorf between 1983 and 1987. (c) Also shown is the correlation of atmospheric 7Be and 210pb. Typical error bars are given as examples.
SCHULER ET AL.' RADIONUCLIDES IN LAKE ZURICH, 1, FLUXES 17,055
J• •L
0
E
0 10 2o 3o 4o 5o
Atmospheric Be-7 Flux [dpm/cn12y]
Fig. 1. (continued)
of 1.65s (95% confidence limits) in particle fluxes are estimated to +10%. Errors in radionuclide fluxes are estimated
from errors in particle flux measurements and an additional 1.650 counting error of generally •5%. Particle fluxes and fluxes of particle-bound 210pb, 7Be, and
137Cs out of the water column, measured at 50 m and 130 m depth, are depicted in Figures 2a-2d, and those of lOBe are shown in Figure 5. Maximum fluxes of 7Be and 210pb are observed in the summer months (July-August) during peaks in primary production and atmospheric deposition of radionuclides. Minimum fluxes were found in the winter
season. This strong seasonality is documented also by the significant correlation of 7Be and 210pb fluxes in the upper water column with water temperature (Table 5). Temperature here is likely a surrogate parameter for light intensity which partially controls primary productivity and, hence, the removal rates of /)articles which are responsible for radionuclide scavenging.
Ttme-w•gh•mi m•an values, are given. tbrors indicated are 1,65 O.
17,056 SCHULER ET AL.' RADIONUCLIDES IN LAKE ZURICH, 1, FLUXES
A striking feature of Figures 2a, 2c and 2d is the enhancement in 210pb and 137Cs fluxes and, to a lesser extent, particle fluxes at the bottom of the lake (sediment trap deployed at 130 m depth) as compared to those in the sediment trap at 50 m,
which probably indicates horizontal transport processes of fine, flocculent particles enriched in trace metals and radionuclides [Santschi, 1989; Schuler et al., 1987]. Irrespective of the various transfer and scavenging processes,
50 m (A) 130 m (B)
a
:t9•3 :t9•4 i• i9•6 :t9•7
,•, 450
'•400
350
300
L>50
:•oo
t50
too
50 m (A) 130 m (B)
Fig. 2. Time series of (a) particle, (b) 7Be, (c) sediment traps moored in Lake Zurich.
b
:.....-.:.:...-.
210pb and (d) 137Cs fluxes in the years 1983-1987 determined in two
SCHULER ET AL.' RADIONUCLIDES IN LAKE ZURICH, 1, FLUXES 17,057
seasonality of atmospheric radionuclide deposition and sediment trap fluxes of 10Be, 7Be, 210pb and particles generally coincide (Figures la and lb, Figures 2b and 2c, Figure 5).
Lead 210. Fluxes of 210pb in the sediment traps are between 50 and 100 % higher in summer than in winter (Table 2) parfly due to higher atmospheric radionuclide deposition and partly due to higher particle fluxes in summer. The decrease in
• 80
E 7O
•' 6O
(• 5O
4O
U_ 3O
I
•_ •0
tO
• 5
E 7
u'•
x to • i-;- 7
5
c,') I
•o o 7
5
50 rn (A) 130 rn (B)
't983 :t984 t985 t986 t987
ii• 50 rn (A) • 130 rn (B)
c
d
t983 t984 t985 ],986 t987
Fig. 2. (continued)
17,058 SCHULER ET AL.: RADIONUCLIDES IN LAKE ZUPdCH, 1, FLUXES
radionuclide fluxes in winter is, however, less than proportional to the decrease in transfer rates of particles in the lake, indicating higher radionuclide scavenging efficiency during the winter season. The average fluxes of 210pb out of the water column (0.94
and 1.22 dpm cm '2 yr '1 at 50 m and 130 m depth, respectively, see Table 2) can be compared to the average atmospheric fallout during 1984-1987 (0.83 dpm cm -2 yr -1) (Table 1). Comparison of the summer atmospheric deposition (1.08 dpm cm'2yr '1) and the sediment trap fluxes (1.18 dpm cm '2 yr '1 at 50 m) indicates the quantitative removal of atmospherically delivered 210pb. During the winter, however, 210pb fluxes in sediment trap A (0.73 dpm cm '2 yr '1) are on average 50% higher than atmospheric deposition (0.48 dpm cm -2 yr'l). The lead 210 supply from the drainage basin and to minor extent remobilization or resuspension of sediments and recycling into the surface waters may increase the isotope flux in the near-surface sediment trap. However, this extra input of 0'25 dpm cm '2 yr' 1 in 0.5 years is _<15% of the annual average input of 210pb to Lake Zurich. The increase in 210pb fluxes with depth is more pronounced
in summer (1.18 dpm cm '2 yr '1 at 50 m and 1.46 dpm cm -2 yr '1 at 130 m) than in winter (0.73 dpm cm '2 yr '1 at 50 m and 0.85 dpm cm '2 yr '1 at 130 m) The larger flux of 210pb in the near-bottom sediment trap during the summer can likely be attributed neither to diffusive remobilization of 210pb into anoxic bottom waters nor to resuspension of underlying sediments. Low current speeds of _<2 cm s' 1 above sediments were measured during lake stratification in a number of Swiss lakes [œemtnin and Itnboden, 1987]. Current speeds of this magnitude are certainly too small to resuspend consolidated sediments. If diffusive remobilization was a relevant process for the enhancement of the 210pb flux, the remobilizafion rate of the iron oxide carrier phase and thus the associated mass flux of iron oxides into the bottom sediment trap should also be significantly increased. In Lake Zurich, iron oxides, however, represent 1-3% of the total mass flux of settling particles throughout the year [Sigg et al., 1987].
Our results are similar to those of van Hoof and Andten [1989], who found large 210pb fluxes in a near-bottom sediment trap deployed in Lake Michigan. However, contrary to our observations, resuspension of sediments during winter and spring and the translocation of nearshore particulates during stratification periods were proposed to be the operative mechanisms of radionuclide transport in this large lake.
The comparison of atmospheric and sedimentary fluxes in Lake Zurich indicates that (1) sediment traps at 50 m depth quantitatively collected the 210pb failing through the water column (differences between atmospheric and sediment trap
5O0
400
300
200
100
0
0
500.
Summer a
ß Trap A (R = 0.77, p < 0.01) : o Trap B (R: 0.75, p < 0.01)
10 20 30 40 50 60
Pb-210 Flux [dpm/m2d]
400
300
200
lOO ,!
0 10 20 30
Winter
, ß Trap A (R = 0.75, p < 0.05)
o TrapB(R 0.85, p<0.01) X
b
4O
Pb-210 Flux [dpm/m2d]
Fig. 3. Correlations of seasonal 210pb and Pb fluxes in sediment traps A and B during 1983/1984. R is the correlation coefficient of the linear regression, and 1-p indicates the confidence level for rejecting the hypothesis that the correlation is zero.
directly applicabie to selected stable trace metals such as Pb. Even though the chemical speciation of 210pb and stable Pb in atmospheric aerosols is different, their prima• mode of input into surface waters is through rain where both 210pb and stable Pb are largely occurring in the dissolved form. Hence, calculations of removal residence times resulting from the
fluxes are likely due to local variations in horizontal presented multitracer study are most likely good estimates for transport); (2) atmospheric fallout is, as expected, the only selected stable trace metals as well. source of 210pb in summer and the dominant source (-67%) in Beryllium 7 and lOBe ' The time series of 7Be fluxes in winter; and (3) vertical fluxes of 210pb increases with depth in sediment traps is presented in Figure 2b. Concurrent minima of the lake, probably due to horizontal inputs. Fluxes of stable Pb atmospheric and sediment trap fluxes in winter are a factor of 2- and 210pb measured during summer and winter 1983 in both 3 lower than the maximum fluxes in summer (Tables 1 and 2). sediment traps are correlated in Figures 3a and 3b. Because of Similar variations in sediment trap fluxes were measured by the close correlation of 210pb and Pb fluxes (r = 0.60-0.85, Dotninik et al. [1989] in 1986 in Lake Geneva. significant at the 99.9% confidence level, Table S), any Average 7Be fluxes of 4.90 dpm cm '2 yr '1 measured in the information gained through the analysis of the investigation near-surface sediment trap A were considerably lower than of radionuclide behavior such as that of 210pb should also be atmospheric inputs (16.0 dpm cm '2 yr '1) (Tables 1 and 2).
SCHULER ET AL.' RADIONUCLIDES IN LAKE ZURICH, 1, FLUXES 17,059
This observation indicates that most 7Be resides in the water column longer than its mean radioactive life of 77 days, thus being removed from the lake mostly by radioactive decay. Time series of 7Be/210pb ratios in atmospheric deposition,
as well as the near-surface and near-bottom sediment traps, are
given in Figure 4. The average 7Be/210pb ratio in the atmospheric deposition rates between 1983 and 1987 is 20.
Even though we assume a relatively large error of <•0% for the ratios in each time interval, Figure 4 reveals a significant seasonal variation of the 7Be/210pb ratio with a maximum in April caused by the spring maximum of 7Be input from the atmosphere and a minimum in late fall. The seasonal cycle of the 7Be/210pb ratio in atmospheric precipitation is, with some delay of 1-2 months, preserved in sediment traps,
sil INIDIjIFIi41AI i,jI,jIAIsloIN1 I,jIF1N!•,INI,jI,j1AIsl IDI I FI i.i I INIjIjIAI i985 i986 t9a7
Fig. 4.
io- Sediment Trap B
4-
i dl FII• I •,114 i dl J I• I sl 0 IN O Id 1Fli. iI•,i14 Ialdl AIsloINID IjIFINIA I 1985 t986 t987
c
I lsloINIDI
Ratio of 7Be and 210pb fluxes measured in (a) atmospheric, Co) sediment trap A and (c) sediment trap B collectors.
17,060 SCHULER ET AL.: RADIONUCLIDES IN LAKE ZURICH, 1, FLUXES
indicating that ?Be and 210pb are removed from the water column by similar transfer mechanisms, i.e., particle scavenging (Figure 4). Because of the shorter removal residence time of 210pb and the decay of ?Be in the water column [Wieland et al., this issue], the ratio of ?Be/210pb is higher in the atmospheric collector than in the sediment traps. Episodic events in particle fluxes with high 210pb and low ?Be concentrations are observed in both sediment traps, possibly indicating the lateral input of older fine particles [Wieland et al., this issue].
Beryllium 10 concentrations measured on settling particles are given in Table 3. The time-dependent fluxes of 10Be and the ratio 10Be/?Be measured on particles which were collected in 1987 are depicted in Figure 5. The average concentration of lOBe in the 50-m and 130-m sediment traps are 12.2 ñ 3.? x 10 ? atoms go1 and 12.2 •: 2.1 x 10 ? atoms g-1 respectively.
Sharma et al. [1987] and Bourles et al. [1984] measured 10Be on marine particles and observed 10Be concentrations ranging from 20 x 107 atoms g-I in the near-surface traps up to 330 x 10 7 atoms g-1 in the near-bottom sediment traps. In Lake Zurich, 10Be concentrations on settling particles agree well with the average 10Be concentrations (14.7 + 2.4 x 107 atoms g-1) on particles which were deposited in the uppermost layers of the sediment between 1960 and 1980 (I. Beer, unpublished results, 1990).
Seasonal variations of 7Be and 10Be fluxes strongly correlate in both sediment traps reflecting the similar seasonality of input functions. Time-weighted mean values of
7Be and 10Be fluxes in the u•er trap are 1.9 x 107 atoms m '2 d '1 and 16.5 x 107 atoms m-" d -1, respectively, and 1.6 x 107 atoms m '2 d -1 and 23.1 x 107 atoms m '2 d -1 in the near-
Period Interval,
days
Dec. 17, 1986 35
Feb. 12, 1987 57
April 3, 1987 50
May 13, 1987 40
June 11, 1987 29
July 1, 1987 20
July 22, 198,7 21 Aug. 12, 1987 21
Sept.. 2, 1987 21
Sept.. 23, 1987 21 Oct.,14, 1987 21
Nov. 10, 1987 27
Dec.. 2, 1987 22
TABLE 3a. 7Be Concentrations and Fluxes in Sediment Traps
7Be 7B½ Flux 7Be 7Be Flux 10Bed7Be in Trap A, in Trap A. in Trap B, in Trap B, in Trap A
10 7 atoms g-1 107atoms m '2 d '1 107 atoms g-I 107atoms ra-2 d-I 0.794-0.04 0.374-0.04 0.414-0.02 0.304-0.03
TABLE 3 b. 10Be Concentrations and Fluxes in Se. din•nt Traps
lOBe 10Be Flux lOBe in Trap A, in Trap A, in Trap B,
10 7 atoms g' 1 107 atoms m '2 d' 1 10 7 atoms g' 1
10Be Flux in Trap B,
10 7 atoms m '2 d' 1
10.524-0.85 11.92+1.50 11.42•0.83
13.94+1.84 13.37+2.19 12.27•0.98
5.374-0.83 14.38+2.58 7.12+1.04
16.61+1.52 14.51+1.96 11.594-2.06
18.304-1.58 33.105:4.28 16.56+2.92
25.42+ 1.96 23.47+2.93 18.60+4.44
12.64+1.35 28.04+4.03 14.124-2.29
7.21+1.07 32.70+__5.59 10.554-0.72
4.964-0.32 16.55+1.93 8.63+0.54
8.084-0.54 10.15+1.19 8.58+0.64
11.894-0.66 8.38•O.94 12.92.•.86
12.254-0.85 10.24+1.22 14.334-0.92
18.914-2.31
17.004-2.11
27.194-4.69
19.68+3.95
46.374-9.38
21.504-5.52
36.20-•.71
36.42+4.38
25.004-2.89
16.18+1.97
16.08+1.89
13.644-1.61
in Trap B
31.054-3.34
18.524-1.77
13.384-2.02
26.784-5.02
10.854-1.99
16.654-4.16
11.914-1.99
10.224-0.91
9.934-0.81
13.094-1.21
16.194-1.29
20.964-1.61
SCHULER ET AL.: RADIONUCLIDES IN LAKE ZURICH, 1, FLUXES 17,061
5O
40
30
ot I I I I JI F M A M
........ L•aP B
ap A
I I I I I I J J A S O N D
1987
• 20
• lO
j! FI MIAIM I jI jIAI S IOINI D 1987
Fig. 5. Fluxes of 10Be in Lake Zurich as well as the ratio 10Be/IBe deposited in sediment traps throughout the year 1987, plotted as a function of time. Errors of 1.65o are given in Table 3.
bottom sediment trap, respectively. The differences in 10Be fluxes between the two traps are significant, i.e., 7.21 + 2.14 x 107 atoms m -2 d -1 (Figure 5). However, 10Be concentrations on settling particles were equal in both sediment traps (Table 3). Hence, the increase in 10Be flux in the lower sediment trap of 40% is consistent with our finding of higher particle, 210pb, and 137Cs fluxes at the bottom of Lake Zurich. Figure 5 also shows the seasonal variation of the 10Be/7Be
ratio in the sediment traps. It further reveals that the average 10Be/7Be ratios in the sediment traps (trap A: 9.1 + 1.5, trap B: 16.6 + 4.2) are increased with respect to atmospheric deposition (1.86 + 0.62) most likely due to decay of 7Be in the water column or some supply of long-lived 10Be from the drainage basin. In the near-surface flap, the ratio remains rather constant (winter average: 11.4 + 3.4, summer average: 7.9 + 0.9). In the near-bottom trap, episodic flux events of 10Be- bearing particles cause two maxima in the 10Be/7Be ratio (26.8 and 16.7) during summer 1987 (average summer ratio: 14.1 + 4.5), illustrating the focusing of particles to the take bottom. The higher 10Be/7Be ratios in winter (Figure 5) mainly reflect some input of allochthonous or resuspended
material as observed in higher 210pb fluxes. Contrary to the 10Be/7Be ratios in the sediment traps, the atmospheric 10Be/7Be ratios show a maximum in summer (May-July) [Luder, 1986] and a minimum in winter (November-January).
With an initial and constant 10Be/7Be ratio in atmospheric deposition of 1.9 and the measured 10Be/7Be ratios in the sediment trap at 50 m water depth, residence times of 7Be in the lake are estimated to be 84 + 7 days in summer 1987 (May- October) and to be 109 + 10 days in winter 1986/1987 (November-April). These values agree well with calculations based on a non steady state model [Wieland et al., this issue]. They also indicate that atmospheric deposition is the main source of 10Be and 7Be in Lake Zurich.
Polonium 210. Polonium 210, which is mainly produced in situ in the water column from 210pb ' is expected to be removed more slowly in lakes [Benoit and Hemond, 1986; Talbot and Andten 1984] and in oceans [e.g., Bacon et al., 1976]. The water overlying the settled particles inside the traps was assessed for its radionuclide content in 1986 and 1987. It was
found that the enrichment of 210po concentrations was significantly higher than in water outside the sediment traps. However, concentrations of 210pb and 7Be were not elevated in water overlying the settled particles, indicating no loss for these nuclides.
While the 210po/210pb ratio of settling particles was 0.3- 0.5, that value almost doubled in all eight cases investigated in 1986 after it was corrected for extra 210po released from the collected particles into the overlying water. When corrected for this artifact, 210po/210pb ratios average 0.4 to 0.7 (see Table 4). These values can be used to estimate residence times
[Wieland et al., this issue]. The appreciable loss of 210po (but not of 210pb ) into the overlying water in the sediment traps is likely due to cell lysis since 210po is mostly taken up into cell interiors [Fisher et al., 1983]. It also suggests a serious possibility of loss of stable trace elements such as Zn, Cd, and Cu, as was previously shown for oceanic conditions by Fowler and Knauer [1986]. Their experiments demonstrated that the loss rates of 210po are g.ood indicators for the release of the labile trace elements Zn, Cd and Cu, which, incidentally, are also taken up into cell interiors [Fisher et al., 1984]. Therefore, previously reported sediment trap fluxes of these elements in Lake Zurich [$igg et al., 1987], which lacked any testing for trace element loss, are most likely only lower limits of the true fluxes.
Cesium 137 The time-dependent 137Cs fluxes measured in the sediment traps are depicted in Figure 2d. Radioactive fallout from the burning Chernobyl reactor increased 137Cs fluxes in the sediment traps from a background value of about 1 dpm m '2 d -1 to 1000 dpm m -2 d '1 during May 1986. Figure 2d reveals that 137Cs is removed from the water column by setfling particles.
Mass balance calculations and the time-dependent inventories of Chernobyl 137Cs in Lake Zurich are discussed by $antschi et al. [1990]. In summer 1986, the 137C s inventory in the water of Lake Zurich decreased from 27 + 3 dpm cm -2 after the fallout event to about 5 dpm cm '2 at the end of the stratification period. Time-integrated 137Cs fluxes in the 50-m sediment trap averaged 8.1 + I dpm cm '2 in 1986, indicating that 137Cs removal by settling particles as shown
17,062 SCHULER ET AL.' RADIONUCLIDES IN LAKE ZURICH, 1, FLUXES
in Figure 2d is a significant process for controlling the isotope activity in the lake. The most striking feature of 137Cs fluxes in sediment traps after the winter of 1986 lake turnover (causing constant 137Cs concentrations in the water) are (1) higher fluxes in the near-bottom sediment trap during summer due to remobilization of !37Cs out of the deepest and seasonally anoxic sediments of the central part of the lake (2) a smaller maximum in !37Cs fluxes during spring and early summer 1987 most likely due to the supply from the drainage basin of allochthonous particulate matter enriched in silicate minerals with high ion exchange capacities for 137Cs. The maximum of the 137Cs flux in May-July 1987 (Figure 2d) does not coincide with the maximum in particle fluxes in April and August-September 1987 (Figure 2a). It indicates that organic matter and calcite originating from primary production which make up most of the mass transfer are not primary carrier
phases for the removal of 137Cs. The correlation coefficients determined from the linear
correlation of the atmospheric fluxes of ?Be and 210pb with their sediment trap fluxes, as well as the coeffients calculated from linear regressions between the atmospheric and sediment trap fluxes of ?Be and 210pb with rainfall, air, and water temperature and the fluxes of stable lead are summarized in Table 5.
Standing Crops of Radionuclides in the Lake Water Results of the determinations of the water column
concentration profiles of ?Be are shown in Figure 6. The winter profile (February 1, 1984) reveals deep penetration of this short-lived nuclide to the bottom waters of the lake. Under
stratified conditions (August 22 and October 10, 1984), the ?Be activity is restricted to the upper layers of the lake.
Lead 210 activities (not shown) were not very precise (0-50% errors) but were low throughout the year (Table 1) and averaged 1-3 dpm per 100 L, resulting in 210pb standing crops of $ and 15 x 10 -2 dpm cm '2 in wintertime and summertime, respectively.
Cesium 13'7 profiles in the water column after Chemobyl fallout are published elsewhere [Santschi et al., 1988, 1990]. Cesium 137 activities in surface waters decreased from 12,000
dpm m '3 in June 1986 to 120 dpm m '3 in November 1988, while hy•olimnetic 137Cs decreased more slowly from ?20 dpm m 'ø in November 1986 to about 480 dpm m '3 in November 1988.
Sediment
Activity profiles of 210pb and 137Cs in a core sampled in 1987 at 137 m water depth are shown in Figure 7. The 137C8 profile reveals two maxima expected from bomb fallout in 1963 and Chernobyl fallout in 1986. The 137Cs inventory from Chernobyl fallout which was calculated from the 134Cs activity in the core amounts to 5.5 + 0.3 dpm cm '2. The sediment inventory is lower than the inventory expected from 137Cs fluxes in the 50-m sediment trap (8.1 -1- 1 dpm cm-2), indicating remobilization of 137Cs from the anoxic sediments of Lake Zurich. Remobilization causes a higher concentration of 137Cs in the seasonally anoxic waters of the hypolimnion as observed in this lake and in Lake Lugano by Santschi et al. [1990].
The 210pb inventory in the sediment core was calculated to be 24.8 -1- 2.6 dpm cm '2 which corresponds to an annual flux of
SCHULER ET AL.' RADIONUCLIDES IN LAKE ZURICH, 1, FLUXES 17,063
TABLE 5. Selected Correlations of Atmospheric and Sediment Trap Fluxes (F) of radionuclides (7Be, 210pb) With Rainfall, Air, and Water Temperature and Fluxes of Stable Lead (Pb).
Correlated Parameters
F(7Be)Atm versus air temperature F(210pb)Atm versus air temperature F(7Be)Atm versus rainfall F(210pb )Atm versus rainfall F(particle) versus water temperature
F(7Be)sTA versus water temperature F(210pb)sTA versus water temperature F(7Be)Atm versus F(2101:•O)Atm F(7Be)sTA versus F(7Be)Atm F(210pb)sT A versus F(2101•O)Atm F(2 !0pb)sTA versus F(Pb)sTA
Yearly Data Summer Data r n r n r
0.78* 23 0.58 11 0.25
0.69* 23 0.59 11 0.18
0.33 22 0.06 12 0.19
0.23 23 0.13 12 0.07
0.59* 54 0.44 + 35 0.03
0.58' 39 0.31 25 0.3 7
0.62* 54 0.59* 35 0.01
0.90* 22 0.91' 11 0.84*
0.77* 19 0.73 + 9 0.85*
0.73* 23 0.70 + 11 0.69 +
0.60* 22 0.77* 14 0.75 +
22 0.75* 14 0.85* F(210pb)sTB versus F(Pb)sTB 0.61 * The correlation coeffwient is denoted as r, and n is the number of values. The subscripts are defined as follows: Atm, atmospheric; STA, sediment trap A; STB, sediment trap B. ß Correlations are significant at the 99.9% confidence level or better. + Correlations are signLficant at the 90% confidence level or better.
Winter Data
n
12
12
11
11
19
14
19
11
10
12
08
08
0 2 4 • 8 10...[øC] 0 4 8 12 16 20. [øC] 0 4 8 12 16 .,Temp;[øC] I ! i I i •_ ! I i , , , "•_ , , , , , ''
Fig. 6. Activity profiles of 7Be in the water of Lake Zurich.
0.78 ñ 0.08 dpm cm '2 yr '1. Hence, average atmospheric deposition between 1983 and 1987 (0.83 dpm cm '2 yr '1) and time,integrated deposition into the sediments agree, probably by coincidence. Measured fluxes of 210pb at 130 m water depth, however, averaged 1.22 dpm m '2 yr' 1 during 1983-1987 (Table 2). It may indicate some loss of 210pb from settling particles through remobilization in this anoxic layer of water. The sedimentation rate derived from the 210pb profde in the sedJxnent core is 0.065 gcm '2 yr '1, while the rate derived from the 1963 peak of the 137Cs profile is lower, i.e., 0.052 gcm '2 yr' 1.
SUMMARY AND CONCLUSIONS
Temporal variations of input and output fluxes of atmospherically delivered radionuclides (10Be, 7Be, 210pb, and Chernobyl 137Cs) have been characterized in order to
assess the efficiency of the transfer from the atmosphere to the sediments. Average fluxes of 7Be and 210pb from the atmosphere measured in D/ibendorf during 1984-1987 amounted to 16 and 0.83 dpm cm '2 yr' 1, respectively, and to 5.0 x 107 atoms m '2 d '1 for !0Be. The maximum in atmospheric fluxes of 7Be and 210pb was observed in summer. However, 7Be/210pb ratios showed, as expected, a maximum in springtime. Input fluxes of 7Be and 210pb from the atmosphere into the
lake were tightly correlated possibly due to temperature control both of radon emanation from soils and of summertime
enhancements of atmospheric removal processes (i.e., thunderstorms).
In Lake Zurich, 7Be and 210pb fluxes at 50 m water depth (sediment trap A) averaged to 4.90 and 0.94 dpm cm '2 yr '1, respectively, and to 6.02 x 106 atoms cm '2 yr '1 for lOBe. At 130 m water depth (sediment trap B) the average fluxes were 3.99 dpm cm '2 yr '1, 1.22 dpm cm '2 yr '1 and 8.43 x 106 atoms
17,064 SCHULER ET AL.: RADIONUCLIDES IN LAKE ZURICH, 1, FLUXF.,S
Activity [dpm/g] .01 .1 1 10 100
i i i • ii lal i ....... ! ........ i .... ,
ß (Pb-210)xs [dpm/g] -o- Cs-137 [10 dpm/g
6
-/ ß Fig. 7. Activity profiles of 210pb and 137Cs in the sediments below the sediment trap array at 137 m water depth.
cm '2 yr -1 for 7Be, 210pb, and 10Be, respectively. This allows three major conclusions. 1. The agreement between atmospheric fluxes of 210pb with
those measured at 50-m depths indicate that 210pb is mainly derived from atmospheric sources. 2. The sediment trap at 50 m water depth is quantitatively
collecting 210pb removed from the water. 3. Beryllium 10 and 210pb fluxes in the sediment trap at 130
m water depth were, on the averal•e, 20% higher than fluxes at 50 m depth due to lateral inputs. Even though 210pb was quantitatively collected in these
traps, this was not the case for 210po. Polonium 210 was found to have been significantly remobilized into the trap solutions, on the average by 50%, during deployment times of 14 to 21 days. Therefore it is likely that previously reported fluxes of labile trace metals such as Zn, Cu, and Cd are only lower limits. Radionuclides such as 210pb can be used as physicochemical tracers for stable trace metals such as Pb in order to evaluate removal processes and residence times.
Acknowledgments. Laboratory work was carried out at the Swiss Federal Institute of Water Resources and Water Pollution Control
(EAWAC0. We thank Klaus Farrenkothen for helping with many asl•CtS of the experimental portion of this study. The manuscript has benefited from critical comments on an earlier draft of the manuscript by D. Imboden (EAWAC•/ETH, Zurich), M. Basksran and O. Benoit
(TAMUG) and J. Dominik (Universit• de Geneve). W. Ela, S. Moran, G. Redden, and two anonymous reviewers kindly provided thoughtful comments on our mansucript. This work was in part supported by EAWAG's teaching and research funds, by the Swiss National Science Foundation, and, at Texas A&M University, Galveston, Texas, by the Texas Advanced Research Program (grant 4697) and the Texas Institute of Oceanography.
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